The present invention relates generally to frequency synthesizers which are used to generate multiple clock frequencies, and more specifically to a frequency synthesizer based upon phase-locked loop technology.
A frequency synthesizer having a phase-locked loop typically includes an input frequency divider for a corresponding input signal and an output frequency divider for its corresponding output signal. At phase-lock, a synthesized frequency f.sub.o is proportional to the input frequency f.sub.i by the relationship shown in equation (1), where M and N are divisors for the input frequency divider and output frequency divider, respectively. ##EQU1## From equation (1), it can be seen that a desired output frequency which is, for example, double the current output frequency is obtained by adjusting variables such as input frequency f.sub.i, divisor M, and/or divisor N. If f.sub.i and M are constant, the dividend N is doubled to obtain the desired output frequency. Alternatively, if f.sub.i and N are normally held constant, the divisor M is halved to obtain the desired output frequency. A wide range of output frequencies may be calculated by varying the values of the divisors M and N in equation (1).
However, the practical limit of the output frequency range is often defined by dynamic loop characteristics underlying the phase-locked loop. The loop characteristics include loop variables such as loop bandwidth, natural frequency, damping factor, among others. Values of the loop characteristics are typically based upon preset parameters of component parts for the phase-locked loop. The preset parameters typically prevent frequency synthesis outside a predefined range of the phase-locked loop.
When a frequency synthesizer attempts to synthesize a frequency within the frequency range of the phase-locked loop, the frequency synthesis will typically succeed. However, when a frequency synthesizer attempts to synthesize a frequency outside the predefined frequency range, the synthesis will often fail.
From the above it is seen that a frequency synthesizer which includes a phase-locked loop adapted to provide a larger range of synthesized frequencies is often desired.